This invention generally relates to imaging apparatus for providing images from digital data and more particularly relates to an apparatus and method for minimizing pixelization effects when forming an image using a spatial light modulator.
Two-dimensional spatial light modulators are being widely used in a range of imaging applications from projection of color images to printing of monochrome and color images onto photosensitive media. Because it forms a complete, two-dimensional image at one time without requiring mechanical movement, the spatial light modulator offers a number of advantages over other types of imaging devices, such as scanning lasers, for example.
A spatial light modulator can be considered essentially as a two-dimensional array of light-valve elements, each element corresponding to an image pixel. Each array element is separately addressable and digitally controlled to modulate light by transmitting or by blocking transmission, or reflection, of incident light from a light source. There are two salient types of spatial light modulators that are being employed for forming images in projection and printing apparatus. The liquid crystal device (LCD) modulates an incident beam by selectively altering the polarization of light for each pixel. An LCD may be transmissive, operating by selectively transmitting the incident beam through individual array elements. Other types of LCD are reflective, selectively changing the polarization of a reflected beam at individual array elements. The second basic type of spatial light modulator currently in use is the digital micromirror device (DMD) as disclosed in U.S. Pat. No. 5,061,049. The DMD modulates by reflection at each individual pixel site.
Spatial light modulators were initially developed for digital projection applications. Examples include display apparatus such as those disclosed in U.S. Pat. No. 5,325,137 to Konno et al. and in U.S. Pat. No. 5,743,610 to Yajima et al. and miniaturized image display, such as mounted within a helmet or supported by eyewear, as is disclosed in U.S. Pat. No. 5,808,800 to Handschy et al.
More recently, spatial light modulators have been used in printing apparatus, from line printing systems such as the printer disclosed in U.S. Pat. No. 5,521,748 (Sarraf) to area printing systems, such as the printer disclosed in U.S. Pat. No. 5,652,661 (Gallipeau et al.)
It is instructive to consider some of the more important differences between projection and printing requirements for spatial light modulator devices. Effective image projection requires that the image forming device provide high levels of brightness. In display presentation, the human eye is relatively insensitive to many types of image artifacts and aberrations, since the displayed image is continually refreshed and is viewed from a distance. Motion and change also help to minimize the effects of many types of image artifacts. High resolution (as typically expressed in pixels per inch) is not a concern for projection applications, with 72 pixels per inch normally satisfactory for many types of images.
Image printing, meanwhile, presents a number of different problems. For example, when viewing output from a high-resolution printing system, the human eye is not nearly as xe2x80x9cforgivingxe2x80x9d to artifacts, aberrations, and non-uniformity, since irregularities in optical response are more readily visible and objectionable on printed output. High resolution may require print output at 2000 dpi (dots per inch) or higher, depending on the application.
For a number of reasons including availability and overall adaptability, LCD spatial light modulators are preferred for both projection and printing applications. However, in spite of some inherent advantages and of continuing development of LCD components, there is room for improvement in LCD imaging performance. Pixelization or gridding effects have been known imaging anomalies where pixel structure is visible in some way, possible when using LCD spatial light modulators in some types of applications. These types of imaging anomalies are primarily due to constraints on pixel fill factor, that is, on the proportion of pixel area that is modulated. Along the periphery of each pixel position is some unused area, which causes pixelization effects. Pixelization was more particularly pronounced in earlier LCD devices that had relatively low fill factors; recent advances in LCD component design are improving fill factor values somewhat. The degree of pixelization or gridding is perceived differently between projection and printing applications, and can be more or less objectionable depending upon the type, color, intensity, magnification, media MTF (Modulation Transfer Function, basically referring to spatial frequency response), media granularity, slope of the media density versus log exposure characteristic, and other characteristics of the image.
Among proposed solutions for correcting pixelization is optical dithering using a blur filter. However, as a solution for image pixelization with spatial light modulators, blur filters have a number of drawbacks. Blur filters can be sizeable and costly to design and manufacture. Blur filters do not provide a flexible solution that can be easily adapted to different imaging conditions or media. A blur filter, such as that proposed for projection systems in U.S. Pat. No. 5,715,029 (Fergason), using birefringence, can have the effect of blurring an image and can exhibit differences in response based on wavelength. Overall, optical solutions are difficult to adjust and are not suitably flexible for dithering in printing system applications.
More elaborate dithering schemes have been proposed for printing apparatus in commonly assigned U.S. patent application Ser. No. 09/630,419, filed Aug. 1, 2000, entitled xe2x80x9cA Method and Apparatus for Printing Monochromatic Imaging Using a Spatial Light Modulator.xe2x80x9d One dithering scheme proposed is to duplicate pixel data by imaging the same data at multiple sites. This method can be used to create multiple overlapped images for the purpose of minimizing effects of pixel site defects in the spatial light modulator. Dithering is also proposed as a method for increasing the effective image resolution. A more complex sequence proposed, loosely termed the xe2x80x9cstop-and-starexe2x80x9d method, uses dithering for resolution enhancement with the following basic sequence: first, with the spatial light modulator in a first position and an initial set of data loaded, source light is applied and modulated in order to form an array of pixels at this first position; then, the spatial light modulator is moved to a second position, new data may be loaded to the device, and source light is again modulated in order to form an array of pixels at this second position. The stop-and-stare method can be used to increase image resolution by a multiple of two or four, depending on the dither pattern. Solutions using the stop-and-stare method have the disadvantage, however, of increasing the amount of data that must be manipulated and exposed or projected, also by a factor of two or four correspondingly. This method can appreciably slow down the imaging process, causing visible flicker for projection applications or reduced throughput in printing applications. Overall, the dither schemes disclosed in the U.S. Ser. No. 09/630,419 application might be appropriate with some types of printing apparatus, where additional resolution is advantageous. However, the schemes disclosed in that application may not be well-suited in other types of projection and printing applications, particularly where LCD devices have linear fill factors in excess of about 80%.
It is worthwhile to be aware of a distinction between linear fill factor from area fill factor. Linear fill factor is measured along a line running through pixel centers in a row of adjacent pixel-modulating sites on the surface of a spatial light modulator. Area fill factor, on the other hand, is measured as the proportion of pixel area within a pixel-modulating site on the surface of a spatial light modulator. Either term may be used with respect to a spatial light modulator.
As was noted above, continuing development of LCD components is improving the fill factor of these devices. Because of this, the dithering approaches outlined in the U.S. Ser. No. 09/630,419 application may not be optimal for some applications using recently developed LCDs having larger fill factor. In some cases, for example, dithering without making corresponding changes in data effectively degrades image quality, so that a blurred, unsatisfactory image could be displayed or printed. Even when pixel data is changed, pixel overlap can have the objectionable effect of reducing image sharpness. For a 4-pixel dithering scheme intended to double resolution in both horizontal and vertical directions, if an LCD has a high fill factor, so that dithering causes a large pixel overlap, the resulting image appears as pixel replication rather than increased resolution. Such a dithered xe2x80x9chigh-resolutionxe2x80x9d image can actually appear to be softer than an undithered image. Under similar conditions, dither can actually degrade an image rather than improve it. This problem can be more or less pronounced depending on factors such as sensitometric characteristics of the media, effective fill factor of the spatial light modulator, and amount of pixel overlap, for example.
Thus, it can be readily appreciated that it would be advantageous to provide a dithering scheme that would eliminate gridding or pixelization effects that betray pixel structure, but where this scheme does not reduce imaging speed. It can also be appreciated that there would be advantages to a dithering method that allows some measure of adjustment and adaptation to suit specific imaging applications.
It is an object of the present invention to provide an imaging apparatus for forming an image from digital data. Briefly, according to one aspect of the present invention an imaging apparatus for forming an image from digital data comprises:
(a) a light source for providing an incident light beam;
(b) a spatial light modulator comprising a planar array of pixel sites, each pixel site capable of selectively modulating said incident light beam according to the digital data in order to form an image pixel;
(c) a movable mounting for retaining said spatial light modulator substantially within a fixed plane, said movable mounting allowing a displacement of said planar array of pixel sites in a direction along said fixed plane to within a predetermined distance between a first position and a second position, wherein said predetermined distance is less than about half the distance between adjacent pixel sites in said direction; and
(d) an actuator coupled to said movable mounting, said actuator providing said displacement of said planar array of pixel sites in said direction.
It is a feature of the present invention that it provides, using the movable mounting and its actuator, a mechanism for repeated incremental movement of the spatial light modulator, thereby increasing the effective area over which each image pixel is formed and effectively increasing the pixel fill factor.
It is an advantage of the present invention that it provides a method for pixel dithering that is inexpensive when compared against pixel replication using conventional optical dithering methods. The method of the present invention also allows adjustment of the amount of and pattern of displacement needed.
It is an advantage of the present invention that it modifies image pixel structure without placing demands on image processing hardware.
It is a further advantage of the present invention that the apparatus and method of the present invention are not resolution-dependent or dependent on pixel size, allowing a range of imaging resolutions to be used requiring a minimum of interaction between dither control hardware and image processing hardware.
It is a further advantage of the present invention that the apparatus and method of the present invention do not negatively impact the writing speed of the printing apparatus.
It is a further advantage of the present invention that it allows some measure of adjustment to compensate for fill factor variability between one imaging apparatus and the next. The present invention allows dithering to provide the maximum effect for eliminating pixelization by using diagonal motion, while having minimal effect on image sharpness.
These and other objects, features, and advantages of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description when taken in conjunction with the drawings wherein there is shown and described an illustrative embodiment of the invention.